This invention relates to making nuclear fuel elements and more particularly to making fuel elements from blocks of graphite having a plurality of fuel chambers as well as a plurality of coolant passageways.
Graphite not only exhibits good structural properties under high-temperature and fast-neutron irradiation conditions, but it is valuable as a moderator in reducing the velocity of fast neutrons. As a result, various nuclear reactors utilize fuel elements made of graphite wherein individual fuel pellets or rods are disposed. One such type of fuel element particularly designed for use in a high-temperature, gas-cooled nuclear reactor is shown in U.S. Pat. No. 3,413,196, issued Nov. 26, 1968.
At one time, it was contemplated that the fuel chambers in such fuel elements could be filled with loose particles of nuclear fuel; however, for safety reasons, it has been decided that the fuel must be bonded together as a cohesive mass so as to prevent its being spread throughout the reactor core should an unforeseen accident occur that might result in rupture of one of the graphite blocks. Different methods of bonding the fuel together have been used; for example, fuel chambers in a graphite block might be filled with a paste-like mixture of coated nuclear particles, a filler such as graphite powder and a binder such as coal tar pitch, by injecting the mixture into the fuel chambers and carbonizing it in situ. Alternatively, individual fuel rods or pellets are often formed in separate molds and then inserted into the fuel chambers either prior to or subsequent to carbonizing. However, both of these methods have had some drawbacks. When fuel elements made in the former manner are exposed to prolonged, fast neutron irradiation, the graphite fuel block shrinks substantially more than the pitch-bonded coated fuel particles, often causing cracking to occur in the structure of the graphite fuel element body. Moreover, if the binder adheres too strongly to the nuclear fuel particle coatings and the binder shrinks substantially more than the coating material, particles may be pulled against one another and ultimately cracked open.
Prospective damage to the nuclear fuel element body as a result of irradiation shrinkage can be avoided by using separate fuel pellets or rods that are unbonded to the interior surfaces of the fuel chambers, which fuel pellets are fabricated separately and then inserted into the chambers. However, this procedure generally requires a gap between the fuel pellet or rod and the interior surface of the fuel chamber, resulting in a considerable penalty in thermal conductivity across this gap in the operating reactor, plus the need to prefabricate the rods or pellets before assembly of the fuel elements. The thermal conductivity penalty is even greater because of the need to allow for manufacturing tolerances, which results in most gaps being slightly larger than needed just to slip the pellets into the fuel chambers. Improvements in the fabrication of nuclear fuel elements of this general type have continuously been sought after.